Automatically actuated pressure relieving spark plugs have been described by Philips (U.S. Pat. No. 4,699,096) and Shifflette (U.S. Pat. No. 5,799,634). The Philips device consists of an unconventionally designed spark plug which incorporates one or more axial passages. These axial passages are outfitted with a tapered shoulder, which acts as a ball seat. A ball is forced against this seat by a helical spring which is oriented axially. Opposite the ball, the helical spring has a spring seat, which is often shown to be adjustable, via an adjusting screw. Excessive cylinder pressure, acting against the ball and its biasing spring, will unseat the ball such that pressure may be relieved upon reaching a predetermined (and adjustable) pressure. As the over-pressure is relieved, or otherwise abated (as at the end of the engine cycle stroke), the ball returns to the seated position by the biasing spring.
Manually induced keyseat style cylinder venting is also disclosed in U.S. Pat. No. 4,326,145 to Foster et. al. (Compression Relief Adapter). One design involves cutting a keyseat-style channel across the spark plug threads. This design develops a robust, pressure holding channel that terminates at a pressure transducer. This design communicates cylinder pressure upward, past the seal, but has no component that is engineered to automatically relieve cylinder pressure upon experiencing excessive temperature or pressure. As such, these designs cannot protect an engine against the deleterious effects of excessive pressure or temperature (or a combination of both) that may be encountered in a running engine.
Detonation will result in an engine at excessive values or combinations of the following: (1) static compression ratio, (2) inlet pressure or boost, (3) advanced ignition timing, (4) lean mixture, (5) low grade fuel. These will be referred to as engine parameters and causes of detonation. Detonation causes cylinder gas temperatures and pressures to greatly exceed normal levels. These are deleterious to engine components, hence unabated detonation will result in engine damage.
The Philips sparkplug reacts to excessive cylinder pressure, characteristic and indicative of detonation. But once the detonating cycle is completed, the vent port in the spark plug is reset, such that the detonating cycle can be repeated. This is, at best, a reactive strategy. Detonation is detected (by pressure which un-scats the ball from its seat) and perhaps abated to some degree by a release of cylinder pressure through the vent port. If the operator is audibly alerted to the venting event, he must then find the cause and correct it without much information available to assist him. One or more of the spark plugs were venting, but the number and location of the detonating cylinders has not been elucidated by the Philips design spark plug.
The design taught in U.S. Pat. No. 5,799,634 to Shifflette also vents in response to excessive cylinder pressure. This design, however, employs permanent deformation of a pressure containing structural member to initiate venting. Once venting is initiated, the spark plug continues to vent cylinder contents until it is repaired or replaced. This device includes multiple stages for the venting of gasses (a small passage, responsive to detonation) or for the venting of liquids (a large passage, responsive to hydrolock). As a detonating condition is developed within the engine from one of the five (5) causes, stated above, this device will respond by forming a small, permanent passage from the combustion chamber to the atmosphere.
The first of the five (5) stated causes of detonation is excessive static compression ratio. In an internal combustion engine, this ratio determines the final compressed pressure (and temperature) of the fuel air mixture. Near the end of the compression stroke, the charge is ignited by the spark. Cylinder pressures and temperatures then increase to effectively drive the piston downward, applying torque to the crankshaft. The proper combustion event is characterized by a flame front progressing through the unburned charge to oxidize the fuel at a finite rate. If the charge is ignited by the spark at excessively high pressures and temperatures the oxidation of the fuel air mixture will occur at an infinite rate. This is descriptive of an instantaneous combustion event and is termed detonation. Detonation in an internal combustion engine can produce instantaneous pressures in excess of 3000 psi, in contrast to the normal operating peak pressure of about 800 psi. Shock waves are generated and gas temperatures increase at a nearly infinite rate. The combustion chamber, valve faces, piston face and cylinder walls arc considerably cooler than the cylinder gas temperature; hence the gas transfers heat to these members at a high rate. This heat transfer to the surrounding surfaces cools the gas, which causes it to contract and decrease in pressure. The thermal energy of the gas is irreversibly lost through heat transfer to the surrounding surfaces. Shock waves, impinging on the surrounding surfaces, lose much of the kinetic energy of the gas to those surfaces. The thermal and kinetic energy of the gas is lost to irreversibilities including heat transfer and sound generation. Engine performance will suffer from detonation. The fuel""s chemical potential energy is released to the cylinder gas, which in turn, irreversibly loses the energy to the surroundings. There is little of the potential energy left over to drive the piston down. The detonation, shock wave generation and abatement and heat transfer occur over only a few degrees of crankshaft revolution; the depleted cylinder gas has no more energy to convert to work.
Detonation is detrimental to engine performance, but it is disastrous to engine components. High pressures can severely overload the structural members of the engine, including the piston, connecting rod, crankshaft, bearings, cylinder head and fasteners. High heat transfer rates will cause localized component temperatures which exceed the melting temperature. Mild detonation is often evidenced by aluminum specks on the insulator of the spark plug. This is caused by the aluminum piston having undergone surface vaporization at the face, and the vaporized aluminum having condensed and solidified on all internal engine surfaces. Since the spark plug insulator is white, the gray aluminum may be easily recognized. Severe detonation will erode a hole in the piston, beat the bearings out of the crankshaft and connecting rods, and break components from structural overload.
In newer engines, detonation has been controlled quite successfully with oxygen sensors and knock sensors. Recently manufactured, well-tuned passenger cars and light trucks seldom suffer from detonation. But a large market segment is not presently served: high performance and racing applications.
Many drag racing applications do not employ oxygen and knock sensors because the technology was not built into the selected powerplant. Aftermarket high-performance accessories can expose a critical weakness of another system which is not capable of preventing detonation. An example is the installation of an aftermarket turbo- or super-charger. The increased air mass entering the cylinder during the intake stroke must be accompanied by proportionally more fuel. If the fuel system is incapable of providing adequate additional fuel, the engine will detonate. During high performance engine tuning, the engine variables are adjusted to produce maximum power. This performance maximum occurs just at the verge of detonation. Maximizing all of the first four (4) of the five (5) engine parameters, listed above, will result in maximum power. Exceeding the limit of any one of the above, or excessive combinations thereof, will result in detonation.
The high performance engine tuner must juggle the engine parameters to produce acceptable power, with a margin of error to the onset of detonation. Such sources of error could include: a drop in ambient temperature, a drop in ambient humidity, engine overheating or xe2x80x9chot-spottingxe2x80x9d, lack of fuel flow (e.g., fuel line restriction, undersized or poor performing fuel pump, exposed fuel pick-up), poor quality fuel, etc. During testing or racing conditions detonation is likely to occur. Sadly, under these conditions, engines are often destroyed in the blink of an eye. Once the engine is running, and large amounts of power are being generated, the tuner/operator has limited control over detonation, short of shutting down the engine. However, this can only occur if the detonation was audible and detected. Shutting the engine down will help to limit the amount of damage done by detonation. Upon subsequent, mandatory engine disassembly and inspection, the locations and causes of detonation may be investigated and a potential cause identified. This will be accompanied by parts replacement.
The permanently deforming pressure relieving spark of this invention is engineered to prevent detonation by reducing compression (ratio). When a cylinder develops excessive pressure or temperature, the spark plug will deform to create an air passage from inside the cylinder to the ambient air outside the cylinder. The air passage allows cylinder contents to escape, thus reducing the internal cylinder pressure. Given that one of the major causes of detonation is high static compression ratio, then a reduction of compression will suppress and eliminate detonation. A simple numerical example would be a 12:1 compression engine which, upon detonation and venting through the spark plug of this invention, would then drop to a lower compression ratio, for instance 10:1. If the charge is leaked (through the spark plug vent port) to atmosphere during the compression stroke, gas pressures and temperatures will not increase as rapidly. Thus, at the instant of charge ignition (by the spark), the temperature and pressure of the charge will be insufficiently high to initiate detonation.
The spark plug of this invention is activated whenever cylinder pressure or cylinder temperature, alone or in combination, exceed predetermined thresholds. Because the spark plug of the present invention is a direct descendant of ordinary, commercially available spark plugs, existing spark plugs may easily be adapted to exhibit pressure and temperature sensitive failure modes. The advantages of modifying existing spark plug designs are manifold. Spark plugs are designed with a number of characteristics which are important to application and operation; thread length (xe2x80x9creachxe2x80x9d), seat (taper or flat with gasket), resistance (radio interference suppression) and heat range comprise just some of these design characteristics. The heat range for any particular spark plug design is principally empirically determined, and standardized experimental procedures can establish the relative (though not absolute) heat range of a spark plug design. Hence, while the spark plugs of different manufacturing origin will have differing heat ranges, and may have very different heat transfer characteristics, the failure mechanisms of this invention may be adapted to any spark plug through the selection of materials with known fusing or melting points and empirically determined pressure sealing characteristics.
The primary activation mechanisms of cylinder pressure and cylinder temperature occur in the ways described below. As they occur, alone or in combination, they cause the spark plug of this invention to permanently deform and form an air passage to vent cylinder contents, thus relieving internal cylinder components of further stresses, and providing a permanent indicator showing which cylinder incurred abnormal operating conditions.
Temperature Activation
The spark plug is located at the hottest point of the engine. This is because it is positioned at the site of the initiation of combustion. The insulator surrounding the center electrode must operate at temperatures between 1300xc2x0 F. and 1800xc2x0 F. Insulator temperatures which drop below this limit will xe2x80x9cfoulxe2x80x9d, with conductive deposits which cannot xe2x80x9cburn off.xe2x80x9d Above the temperature range limit, the insulator becomes sufficiently hot to ignite the charge upon compression, and prior to the spark. This condition is known as pre-ignition and may very well result in detonation.
The spark plug body acts as a heat sink for the insulator. The rate of heat transfer from the insulator to this body will determine the operating temperature of the insulator. The cylinder head and coolant (if present) act as a heat sink for the spark plug body. Thus, in a normally operating engine, heat is transferred from the insulator, through the spark plug body, to the cylinder head, and finally to the coolant. Engine overheating may occur from many causes, but the coolant and cylinder head will be among the first to exhibit abnormally elevated temperatures. As the cylinder head temperature increases, the spark plug temperature will also increase. Hence the spark plug body is an accurate indicator of engine operating temperature and pending overheating.
A pressure relief port in the side wall of the spark plug body may be plugged with a sealing element which is affixed by a fusible metal (or solder, plastic or any other solid) having a known softening or melting point. Once the spark plug body adjacent to this fusible metal attains a predetermined temperature, the metal will soften or liquefy, freeing the scaling element and permitting it to be ejected by the pressure within the cylinder. This results in the opening of the pressure relief port in the spark plug.
Pressure Activation
Detonation and hydrostatic lock are leading causes of cylinder overpressure. Although they are very different phenomena, their results arc equally disastrous. High cylinder pressures will break, buckle or rupture engine components. The spark plug of this invention responds to cylinder overpressures by opening the vent port. The structural element acting to contain a scaling element and cylinder pressure must be designed to create an opening at a known maximum cylinder pressure limit.
As previously described, detonation is a high-temperature event. Hydrostatic lock, on the other hand, will occur at comparatively low temperatures. Liquid water must be below 100xc2x0 C. (212xc2x0 F.) at atmospheric pressure. The liquid in the cylinder will become a remarkable heat sink; cylinder and engine component temperatures will plummet as liquid water is ingested, making hydrolock a low engine temperature phenomenon.
Combination Pressure and Temperature Activation
Pressure and temperature within the cylinder are independent properties. Pressure and shock waves, if present, will pervade throughout the cylinder and combustion chamber. The upshot of this is that the pressure exerted upon the piston face is materially identical to the pressure to which the spark plug (and its weakened structure) is exposed. Detonation generates high rates of temperature increase in the cylinder gas; this high temperature effects a high heat transfer rate to all engine components exposed to the hot gas. Instantaneous component temperatures will rise rapidly, although not evenly, throughout the cylinder. Thermal diffusivity (k/xcfx81cp), component mass and conductive boundary conditions (heat sinking ability) also determine a component""s temperature change with time. During detonation the first component to exhibit marked temperature rise is the center electrode and internal insulator of the spark plug. The piston face will also show a sharp temperature rise because it has a high exposed surface for convective, conductive and radiative heat transfer, it is subject to high thermal diffusivity, it has a low component mass, and it is a modest heat sink, being able to diffuse heat only to cylinder walls, piston pin, and connecting rod. The spark plug, per se, will be among the first engine components to show marked temperature rise from detonation.
As metals and alloys are heated to near melting temperature a reduction in strength is nearly always observed. Put simply, high temperatures degrade the strength of metallic components. One likely class of metal alloys suitable for use as pressure containing structural elements of a spark plug is solders. Depending upon composition, solders exhibit either (1) melting points, (2) eutectics, or (3) liquidusxe2x80x94solidus temperatures. Regardless of the phenomenological phase change characteristics, solders exhibit decreased strength (tensile and shear) at temperatures elevated to nearly the xe2x80x9cmeltxe2x80x9d, temperature. Solders xe2x80x9csoftenxe2x80x9d, as the melt temperature is approached.
A detonating engine will increase the spark plug temperature, and dramatically increase the cylinder pressure. Either of these events, taken individually, might not be sufficient to cause the pressure containing structural element to deform and allow cylinder venting. However, the combination of high cylinder pressure along with a softening of the structural sealing element (nearing its melting point) will be sufficient to cause the spark plug to vent. The combined temperature and pressure effects, characteristic of detonation may be collectively employed to initiate spark plug venting.
Temperature Indicating and Monitoring
The body of a spark plug, which is in thermal contact with the insulator and the cylinder head, is a reliable indicator of engine temperature and operating condition. Because of this, much effort has been expended within the industry to place thermocouple probes on and within the spark plug. By and large, such efforts have been unsuccessful. It has proved difficult, or expensive, or both, to place and maintain thermocouples on a spark plug of an operating engine.
Several commercially available products will provide spark plug temperature monitoring at very little cost and effort. Temperature indicating strips, consisting of a number of temperature panels mounted to the body of a spark plug, will blacken or change color sequentially as temperature rises. A spark plug may be monitored for temperature simply by affixing such a temperature indicating strip to the appropriate location on the spark plug body. Temperature indicating paints may also be applied to the spark plug body in a temperature indicating matrix, such that maximum attained temperature is easily determined upon spark plug removal and/or inspection.
Maximum Temperature and Maximum Pressure Indicating and Monitoring
The spark plug of this invention can be used to indicate an engine""s operating temperature and cylinder pressure. Once the values have been determined, the properly selected spark plug will stop detonation at its onset.
A sparkplug having xe2x80x9caveragexe2x80x9d,temperature and pressure failure thresholds is selected and installed, and the engine is operated. If the plug should vent, the maximum temperature reached should be recorded and the spark plug having the next highest temperature and pressure failure thresholds should be installed, and the engine operated. If the plug should vent, the preceding steps are repeated until a plug is used that does not vent when the engine is operated.
If the plug does not vent, the maximum temperature reached should be recorded and correlated to a xe2x80x9cfailurexe2x80x9d,pressure, based upon the spark plug""s predetermined operating characteristics. One may then conclude that the cylinder pressure did not exceed the maximum failure pressure. The plug may then be replaced by the spark plug having the next lower temperature and pressure failure thresholds, and the engine again operated. This procedure should be repeated using spark plugs having lower failure thresholds until a plug is found to have failed during operation.
A comparison of the pressure and temperature parameters of a failed spark plug and the like parameters of the closest spark plug that did not fail will indicate a narrow range of pressure and temperature within which the engine normally operates. Thereafter, should detonation occur, the spark plug will vent, relieving detonation, and the tuner may then investigate the cause of detonation. In this manner the tuner may determine important engine operating conditions without the need for expensive or cumbersome instrumentation. The system described herein provides a very inexpensive methodology for deducing the maximum engine operating pressure and spark plug temperature. Once the proper spark plug has been selected for an application, that spark plug will provide ongoing protection against detonation, maximized protection against mild hydrolock, maximum pressure monitoring, and maximum temperature monitoring.
Accordingly, it is an object of the present invention to design a permanently deforming spark plug that will create an air vent to bleed off excess internal cylinder pressure upon the onset of predetermined conditions of temperature and pressure. It is a further object of the invention to design a spark plug that will protect internal engine components by relieving excess cylinder pressure whenever such pressure exceeds a predetermined maximum threshold. It is yet another object of the invention to provide a spark plug suitable for engine diagnosis that will provide an indication of maximum temperature reached during engine operation and will further indicate whether engine pressure exceeded a known threshold. It is still a further object of the invention to provide a permanently deforming spark plug whose failure modes are independent of parameters such as length of reach, type of seat, or operational temperature range. These and other objects of the invention will become evident through the following explanation of the invention.